U.S. patent application number 16/649108 was filed with the patent office on 2020-07-16 for cellulose nanofiber, sheet-like material obtained therefrom, and method for producing cellulose nanofiber and sheet-like materia.
This patent application is currently assigned to NATIONAL UNIVERSITY CORPORATION OITA UNIVERSITY. The applicant listed for this patent is NATIONAL UNIVERSITY CORPORATION OITA UNIVERSITY. Invention is credited to Taro KINUMOTO.
Application Number | 20200224365 16/649108 |
Document ID | / |
Family ID | 67218533 |
Filed Date | 2020-07-16 |
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United States Patent
Application |
20200224365 |
Kind Code |
A1 |
KINUMOTO; Taro |
July 16, 2020 |
CELLULOSE NANOFIBER, SHEET-LIKE MATERIAL OBTAINED THEREFROM, AND
METHOD FOR PRODUCING CELLULOSE NANOFIBER AND SHEET-LIKE
MATERIAL
Abstract
The present invention provides: a cellulose nanofiber enabling
the provision of a high-performance sheet-like material; a method
for producing the cellulose nanofiber; and a sheet-like material
obtained from the cellulose nanofiber. A bamboo-derived cellulose
nanofiber having a cellulose purity of at least 90%, a fiber
diameter of 10-20 nm, and a crystallinity of at least 70% can be
obtained by a method comprising: (1) a step for subjecting a bamboo
material to an alkali treatment and a mechanical treatment to
prepare bamboo fibers; (2) a step for delignificating the obtained
bamboo fibers; (3) a step for mechanically spreading the
delignificated bamboo fibers; (4) a step for removing hemicellulose
from the spread bamboo fibers; and (5) a step for removing metal
components from the bamboo fibers from which hemicellulose has been
removed. A high strength sheet material having a tensile strength
of 7-200 N for a basis weight of 10-210 g/m.sup.2 or a high
strength sheet material having a tensile strength of 7-200 N for a
density of 0.3-1.1 g/cm.sup.3 can be obtained by making this
cellulose nanofiber into a sheet.
Inventors: |
KINUMOTO; Taro; (Oita-shi,
Oita, JP) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
NATIONAL UNIVERSITY CORPORATION OITA UNIVERSITY |
Oita-shi, Oita |
|
JP |
|
|
Assignee: |
NATIONAL UNIVERSITY CORPORATION
OITA UNIVERSITY
Oita-shi, Oita
JP
|
Family ID: |
67218533 |
Appl. No.: |
16/649108 |
Filed: |
March 12, 2018 |
PCT Filed: |
March 12, 2018 |
PCT NO: |
PCT/JP2018/009537 |
371 Date: |
March 19, 2020 |
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
D21B 1/021 20130101;
D21B 1/06 20130101; D21H 11/12 20130101; D21H 11/18 20130101; D21H
15/02 20130101 |
International
Class: |
D21B 1/02 20060101
D21B001/02; D21H 11/12 20060101 D21H011/12; D21B 1/06 20060101
D21B001/06 |
Foreign Application Data
Date |
Code |
Application Number |
Jan 10, 2018 |
JP |
2018-002149 |
Claims
1. A bamboo-derived cellulose nanofiber, wherein the bamboo-derived
cellulose nanofiber has a cellulose purity of greater than or equal
to 90%, a fiber diameter of 10 to 20 nm, and a crystallinity of
greater than or equal to 70%.
2. A suspension wherein the bamboo-derived cellulose nanofibers
according to claim 1 are dispersed in water.
3. A suspension wherein the bamboo-derived cellulose nanofibers
according to claim 1 are dispersed in an organic solvent.
4. A method for manufacturing bamboo-derived cellulose nanofibers,
the method comprising the following steps (1) to (5): (1) applying
alkali treatment and mechanical treatment to a bamboo material to
create bamboo fibers, (2) applying delignification treatment to the
obtained bamboo fibers, (3) mechanically defibrating the bamboo
fibers having undergone the delignification treatment, (4) removing
hemicellulose from the defibrated bamboo fibers, and (5) removing
metal components from the bamboo fibers after the removal of
hemicellulose.
5. The method according to claim 4, wherein a chipped bamboo
material with the inner and outer skins removed is used.
6. The method according to claim 5, wherein the chipped bamboo
material has a length of 1 to 10 cm.
7. The method according to claim 4, wherein the alkali treatment of
the bamboo material is performed using sodium hydroxide.
8. The method according to claim 4, wherein the mechanical
treatment of the bamboo material is performed using a mixer.
9. The method according to claim 4, wherein the delignification
treatment of the bamboo fibers is performed using at least one
solvent of peracetic acid, chlorous acid, sodium sulfite, sulfuric
acid, ozone, an enzyme, and a microorganism (bacteria).
10. The method according to claim 4, wherein the removal of
hemicellulose is performed using an aqueous solution of potassium
hydroxide.
11. The method according to claim 4, wherein the removal of the
metal components is performed using an acidic solution.
12. The method of claim 11, wherein a hydrochloric acid solution is
used as the acidic solution.
13. A sheet material comprising bamboo-derived cellulose
nanofibers, wherein the sheet material exhibits a tensile strength
of 7 to 200 N per basis weight of 10 to 210 g/m.sup.2.
14. A sheet material comprising bamboo-derived cellulose
nanofibers, wherein the sheet material exhibits a tensile strength
of 7 to 200 N per density of 0.3 to 1.1 g/cm.sup.3.
15. A method for manufacturing a sheet material comprising
bamboo-derived cellulose nanofibers, wherein bamboo-derived
cellulose nanofibers obtained by the method according to claim 4
are formed into a sheet.
16. The method according to claim 15 wherein the sheet is formed
by: (a) creating a suspension comprising bamboo-derived cellulose
nanofibers dispersed in water, (b) removing the water from the
suspension and collecting the residue thereof, 18 and (c) applying
hot press treatment to the collected residue to obtain the sheet
material.
17. The method according to claim 16, wherein the sheet is formed
by collecting cellulose nanofibers from the suspension of (a),
dispersing the collected cellulose nanofibers in an alcohol to make
a separate suspension, and applying hot press treatment
thereto.
18. The method according to claim 15, wherein the sheet is formed
by: (a) creating a suspension comprising cellulose nanofibers
dispersed in an alcohol, (b) spreading the suspension on a
substrate to form a film, and (c) applying freeze-drying treatment
to the film of the suspension to obtain the sheet material.
19. A bamboo-derived lignocellulose nanofiber, which is
manufactured by the method according to claim 4, and has a lignin
content of 1 to 2 wt %.
Description
FIELD
[0001] The present invention relates to a cellulose nanofiber, or
more specifically, a cellulose nanofiber obtained from bamboo as a
raw material, a sheet-like material comprising the same, and
manufacturing methods thereof. Furthermore, the present invention
relates to a nanofiber known as "lignocellulose nanofiber", which
includes a small amount of lignin and is obtained by the above
manufacturing method.
BACKGROUND
[0002] In recent years, cellulose nanofibers using bamboo as a raw
material have been focused on in a wide range of fields, including
reinforcement materials for plastics, solar cells and medicine, and
sheet-like materials manufactured using these cellulose nanofibers
as a raw material are receiving more attention.
[0003] Conventionally, conifer pulp was primarily used as the raw
material for cellulose nanofibers. In recent years, in addition to
conifers, bamboo has been used as a raw material to manufacture
cellulose nanofibers.
[0004] For example, PTL 1 discloses a composite material obtained
from bamboo-derived cellulose nanofibers and having excellent
conductivity, high tensile strength and high tensile modulus, and a
manufacturing method thereof.
[0005] PTL 2 describes cellulose nanofibers with a diameter of
about 50 nm, and a manufacturing method thereof, and indicates
bamboo as well as various other types of plant material as a
cellulose raw material.
[0006] PTL 3 describes a method for manufacturing fine fibrous
cellulose, and indicates bamboo as well as various other types of
plant material as a cellulose raw material.
[0007] Methods for manufacturing cellulose nanofibers include
mechanical defibration methods and chemical defibration methods.
Manufacturing cellulose nanofibers by a mechanical defibration
method using conifers or bamboo as a raw material tends to result
in low crystallinity. Industrial products which use bamboo as a raw
material are provided by, for example, Chuetsu Pulp & Paper
Co., Ltd. and the manufacture thereof is performed via a mechanical
defibration method.
[0008] The cellulose nanofibers known to date feature a purity of,
at most, about 87%, a cellulose crystallinity of, at most, about
66%, and an aspect ratio of, at most, about 100. In order to obtain
sheet-like materials with higher performance, it is necessary to
improve the characteristics of cellulose nanofibers.
PRIOR ART DOCUMENTS
Patent Literatures
[PTL 1] JP 2017-115069 A
[PTL 2] JP 5910504 B2
[PTL 3] JP 2012-012713 A
SUMMARY
Technical Problem
[0009] Out of consideration of the above circumstances, the present
invention has an object of providing a cellulose nanofiber that
enables provision of a high-performing sheet-like material, a
method for manufacturing the same, and a sheet-like material
obtained from the cellulose nanofiber. An additional object is to
provide a nanofiber known as "lignocellulose nanofiber", which is
obtained from the above manufacturing method and contains a small
amount of lignin.
Solution to Problem
[0010] The present inventors, through research of cellulose
nanofibers using bamboo as a raw material in place of conifer pulp,
were able to obtain cellulose nanofibers having a cellulose purity
of greater than or equal to 90%, a fiber diameter of about 10 to 20
nm, and a crystallinity of greater than or equal to 70% by
performing both a relatively mild mechanical defibration method
(using a mixer) and a multi-stage chemical defibration method,
thereby completing the present invention.
[0011] More specifically, the bamboo-derived cellulose nanofiber of
the present invention is characterized as having a cellulose purity
of not less than 90%, a fiber diameter of 10 to 20 nm, and a
crystallinity of not less than 70%.
[0012] The bamboo-derived cellulose nanofiber of the present
invention can be obtained according to the manufacturing method
characterized by comprising the following steps (1) to (5): [0013]
(1) applying alkali treatment and mechanical treatment to a bamboo
material to create bamboo fibers, [0014] (2) applying
delignification treatment to the obtained bamboo fibers, [0015] (3)
mechanically defibrating the bamboo fibers having undergone the
delignification treatment, [0016] (4) removing hemicellulose from
the defibrated bamboo fibers, and [0017] (5) removing metal
components from the bamboo fibers after the removal of
hemicellulose.
[0018] The sheet-like material comprising the bamboo-derived
cellulose nanofibers according to the present invention is
characterized by exhibiting a tensile strength of 7 to 200 N per
basis weight of 10 to 210 g/cm.sup.2, and a tensile strength of 7
to 200 N per density of 0.3 to 1.1 g/cm.sup.3.
[0019] The sheet-like material comprising the bamboo-derived
cellulose nanofibers according to the present invention can be
obtained by a manufacturing method in which the bamboo-derived
cellulose nanofibers according to the present invention are formed
into a sheet.
[0020] The sheet can be formed according to a method of removing
the dispersion medium from a suspension of cellulose nanofibers.
The method for removing the dispersion medium can be, for example,
natural drying, hot press treatment, or freeze-drying. The
dispersion medium can be water or an organic solvent, and the
organic solvent can be an alcohol, for example.
[0021] In the case of hot press treatment, cellulose nanofibers are
formed into a sheet preferably by:
(a) creating a suspension comprising bamboo-derived cellulose
nanofibers dispersed in water (b) removing the water from the
suspension and collecting the residue thereof, and (c) applying hot
press treatment to the collected residue to obtain a sheet-like
material.
[0022] The sheet-like material may be obtained by collecting the
cellulose nanofibers from the suspension of (a) above, dispersing
the collected cellulose nanofibers in an alcohol to make a separate
suspension, and applying hot press treatment thereto.
[0023] In the case of freeze-drying, cellulose nanofibers can be
formed into a sheet preferably by:
(a) creating a suspension comprising cellulose nanofibers dispersed
in an alcohol, (b) spreading the suspension on a substrate to form
a film, and (c) applying freeze-drying treatment to the film of the
suspension to obtain a sheet-like material.
[0024] Additionally, the bamboo-derived lignocellulose nanofiber
according to the present invention has a lignin content of about 1
to 2 wt %, and is obtained by stopping the delignification
treatment of the step (2) of the above method for manufacturing
bamboo-derived cellulose nanofibers at the time when a
predetermined amount of lignin is obtained.
Advantageous Effects of Invention
[0025] The present invention enables improvement in the performance
of bamboo-derived cellulose nanofibers and use of a strong
sheet-like material using the bamboo-derived cellulose nanofibers
as a raw material. Thus, the present invention can be applied to
new uses thereof.
[0026] Additionally, the bamboo-derived lignocellulose fibers of
the present invention mixed with a resin can be expected to form a
composite material (for example, a composite material for
automobiles or home appliances) which is more useful than a
composite material using high-strength cellulose nanofibers having
reduced lignin content.
BRIEF DESCRIPTION OF DRAWINGS
[0027] FIG. 1 is a graph showing the content and extraction rate of
bamboo-fiber lignin relative to time of treatment by peracetic
acid.
[0028] FIG. 2 is a graph showing the content and extraction rate of
bamboo-fiber hemicellulose after the peracetic acid treatment
relative to the time of treatment.
[0029] FIG. 3 is a graph showing the relationship between KOH
concentration in an aqueous solution and the amount and extraction
rate of bamboo-fiber hemicellulose.
[0030] FIG. 4 is a graph showing the relationship between amount of
an aqueous KOH solution and the content and extraction rate of
bamboo-fiber hemicellulose.
[0031] FIG. 5 is a graph showing the relationship between the
treatment temperature for removing hemicellulose and the content
and extraction rate of hemicellulose.
[0032] FIG. 6 shows FE-SEM results for bamboo fibers before
delignification treatment with (a) an FE-SEM image and (b) a graph
showing fiber distribution.
[0033] FIG. 7 shows FE-SEM results for bamboo fibers 1 hour after
delignification treatment with (a) an FE-SEM image and (b) a graph
showing fiber distribution.
[0034] FIG. 8 shows FE-SEM results for bamboo fibers 3 hours after
delignification treatment with (a) an FE-SEM image and (b) a graph
showing fiber distribution.
[0035] FIG. 9 shows FE-SEM results for bamboo fibers 6 hours after
delignification treatment with (a) an FE-SEM image and (b) a graph
showing fiber distribution.
[0036] FIG. 10 shows FE-SEM results for bamboo fibers 8 hours after
delignification treatment with (a) an FE-SEM image and (b) a graph
showing fiber distribution.
[0037] FIG. 11 shows FE-SEM results for bamboo fibers after
hemicellulose removal with (a) an FE-SEM image and (b) a graph
showing fiber distribution.
[0038] FIG. 12 shows FT-IR spectra of cellulose nanofiber sheet
according to the present invention.
[0039] FIG. 13 shows electron microscopy results for a cellulose
nanofiber sheet created by hot pressing with water as a dispersion
medium with (a) a TEM image and (b) an FE-SEM image and the insert
showing an external appearance of the sheet.
[0040] FIG. 14 shows electron microscopy results for a cellulose
nanofiber sheet created by hot pressing with ethanol as a
dispersion medium with (a) a TEM image and (b) an FE-SEM image and
the insert showing an external appearance of the sheet.
[0041] FIG. 15 shows electron microscopy results for a cellulose
nanofiber sheet created by freeze-drying with (a) a TEM image and
(b) an FE-SEM image and the insert showing an external appearance
of the sheet.
[0042] FIG. 16 is an XRD pattern of one sample of cellulose
nanofiber sheet according to the present invention.
[0043] FIG. 17 is an XRD pattern of another sample of a cellulose
nanofiber sheet according to the present invention.
[0044] FIG. 18 is an XRD pattern of yet another sample of a
cellulose nanofiber sheet according to the present invention.
[0045] FIG. 19 is a gas desorption-adsorption isotherm of the
cellulose nanofiber sheets according to the present invention.
[0046] FIG. 20 is a graph showing the pore diameter distribution
for a cellulose nanofiber sheet according to the present invention
created by hot pressing with water as a dispersion medium.
[0047] FIG. 21 is a graph showing the pore diameter distribution of
a cellulose nanofiber sheet according to the present invention
created by hot pressing with ethanol as a dispersion medium.
[0048] FIG. 22 is a graph showing the pore diameter distribution of
a cellulose nanofiber sheet according to the present invention
created by freeze-drying.
[0049] FIG. 23 is a graph showing the tensile strengths relative to
the mass of a cellulose nanofiber sheet according to the present
invention created by hot pressing with water as a dispersion medium
and of comparative sheets.
[0050] FIG. 24 is a graph showing the tensile strengths relative to
the thickness of a cellulose nanofiber sheet according to the
present invention created by hot pressing with water as a
dispersion medium and of comparative sheets.
[0051] FIG. 25 is a graph showing the tensile strengths relative to
the density of a cellulose nanofiber sheet according to the present
invention created by hot pressing with water as a dispersion medium
and of comparative sheets.
[0052] FIG. 26 is a graph showing the tensile strengths relative to
the basis weight of a cellulose nanofiber sheet according to the
present invention created by hot pressing with water as a
dispersion medium and of comparative sheets.
[0053] FIG. 27 shows the fibers in a cellulose nanofiber sheet
according to the present invention with (a) an FE-SEM image and (b)
a graph showing fiber distribution.
[0054] FIG. 28 shows the fibers in a sheet created with Celish.TM.
with (a) an FE-SEM image and (b) a graph showing fiber
distribution.
DESCRIPTION OF EMBODIMENTS
[0055] To obtain a sheet-like material comprising bamboo-derived
cellulose nanofibers, it is necessary to manufacture the
bamboo-derived cellulose nanofibers as the raw material thereof.
According to the present invention, by using a combination of a
relatively mild mechanical defibration method (using a mixer) and a
multi-stage chemical defibration method, cellulose nanofibers
exhibiting improved characteristics relative to conventional
cellulose nanofibers can be manufactured.
[0056] More specifically, the method for manufacturing the
bamboo-derived cellulose nanofiber according to the present
invention includes the following steps (1) to (5): [0057] (1)
applying alkali treatment and mechanical treatment to a bamboo
material to create bamboo fibers, [0058] (2) applying
delignification treatment to the obtained bamboo fibers, [0059] (3)
mechanically defibrating the bamboo fibers having undergone the
delignification treatment, [0060] (4) removing hemicellulose from
the defibrated bamboo fibers, and [0061] (5) removing metal
components from the bamboo fibers after the removal of
hemicellulose.
[0062] In the step (1) of manufacturing bamboo fibers, the alkali
treatment and mechanical treatment are used to create bamboo fibers
from a bamboo material.
[0063] The bamboo material used in the present invention is not
particularly limited, and can be any plant that includes so-called
bamboo fibers, such as moso bamboo, madake bamboo, black bamboo, or
shinotake bamboo.
[0064] In order to improve the effect of the alkali treatment and
the purity of obtained fibers, it is preferable to remove the inner
and outer skins of the bamboo material in advance. With the goal of
manufacturing fibers with uniform diameters, it is more preferable
to remove the inner and outer skins such that only a portion where
the fiber bundles of the bamboo material used are uniform is
left.
[0065] It is also preferable to loosen the bamboo material by
pressure rolling (pressing treatment) with pinch rollers preset
with a peripheral speed difference before performing the alkali
solution treatment. By so doing, the infiltration rate of the
alkali solution increases and infiltration becomes uniform, so that
the separation removal rate of the lignin and the hemicellulose in
the subsequent alkali treatment can be increased. For this purpose,
it is also possible to, for example, employ a treatment that uses a
hydraulic press or a treatment that uses a roller.
[0066] Additionally, since the treatment effect is reduced if the
bamboo material is dry when undergoing the treatment using an
alkali solution, it is preferable to store the bamboo material in a
solution, or in a freezer or refrigerator, without letting the
bamboo material dry by the start of the treatment. More preferably,
in order to suppress the proliferation of bacteria, the bamboo
material can be soaked in a liquid effective for that purpose, such
as an aqueous solution of hydrogen peroxide, perchloric acid, or
sulfuric acid, and stored in a refrigerator. From the perspective
of safety and waste products, it is most preferable to use hydrogen
peroxide.
[0067] The bamboo material to undergo alkali treatment is
appropriately cut as needed for the volume of the treatment vessel.
In order to increase the treatment efficiency, in the present
invention, it is preferable to use a chipped bamboo material cut
to, for example, approximately 1 to 10 cm in length.
[0068] The alkali treatment can be performed by soaking the bamboo
chips in an alkali solution such as an aqueous solution of sodium
hydroxide, sodium bicarbonate, or potassium hydroxide. If an
aqueous solution of sodium hydroxide is used, from the perspective
of efficiency, the concentration of the aqueous solution is
preferably 0.01 to 1.00 M, more preferably 0.10 to 1.00 M, or still
more preferably 0.10 to 0.50 M. The treatment temperature is
preferably 30 to 200.degree. C., more preferably 50 to 150.degree.
C., or still more preferably 100 to 150.degree. C. The treatment
pressure is preferably 101 to 500 kPa, or more preferably 101 to
200 kPa. The treatment time is preferably 1 to 3 hours, or more
preferably 3 hours. The alkali-treated bamboo chips are removed
from the alkali solution and washed with water. The washing should
continue until the effluent water is neutral.
[0069] Next, in order to obtain bamboo fibers, the bamboo chips are
mechanically treated. This treatment can be performed using a
general mixer to stir the bamboo chips with water at room
temperature. The type of mixer used is not particularly limited as
long as it can decompose the bamboo chips into a fibrous state. The
treatment conditions can be appropriately determined such that a
desired treatment effect is obtained. By drying after the
treatment, bamboo fibers are obtained.
[0070] The delignification treatment step (2) can be performed by
contacting the bamboo fibers obtained in step (1) with a
delignification treatment solution. The delignification treatment
solution can be a solution of, for example, acetic acid, chlorous
acid, sodium sulfite, sulfuric acid, ozone, an enzyme, or a
microorganism (bacteria). After dispersing the bamboo fibers in the
delignification treatment solution and then letting the fibers sit,
the fibers are separated from the treatment solution, and then
washed and dried, whereby delignified bamboo fibers can be
obtained. The delignification treatment solution can be used at a
temperature of, for example, room temperature to 220.degree. C., or
preferably approximately 60 to 100.degree. C. The sitting time is
preferably 1 to 8 hours, more preferably 1 to 6 hours, or still
more preferably 3 to 6 hours.
[0071] The longer the treatment time, the higher the lignin
extraction rate. For example, in the case of using peracetic acid
(volume ratio of acetic acid:hydrogen peroxide is 1:1) and a
sitting temperature of 80.degree. C., at 6 hours of treatment, the
extraction rate was 100% and white fibers were obtained. In this
case, almost no hemicellulose was extracted. The longer the
treatment time, the smaller the diameter of the bamboo fibers. At 6
hours of treatment, fibers with an average diameter of about 16 nm
were obtained.
[0072] The bamboo-derived lignocellulose nanofibers of the present
invention, in which the lignin content is 1 to 2 wt %, are obtained
by stopping the sitting of bamboo fibers in the delignification
treatment solution in the step of delignification treatment (2) at
the time when a predetermined lignin content is obtained. In the
case above, the time of sitting using peracetic acid at 80.degree.
C. as mentioned above can be, for example, about 0.5 to 2 hours or
about 0.5 to 1.5 hours. Except for the sitting time in step (2),
the bamboo-derived lignocellulose nanofibers of the present
invention can be manufactured by the same method as manufacturing
the bamboo-derived cellulose nanofibers of the present
invention.
[0073] In the case of using peracetic acid as the treatment
solution, it is considered that the aromatic ring of lignin
undergoes cleavage as shown below, and lignin is removed from
bamboo fibers (see Hyoe Hatakeyama, Japan TAPPI Journal, Vol. 20,
No. 11, p. 15 (1966)).
##STR00001##
[0075] The mechanical defibration step (3) of the delignified
bamboo fibers can be performed by stirring the bamboo fibers with
water in a mixer. As long as defibration is not hindered by the
mixing, the type of mixer is not particularly limited. From the
perspective treatment efficiency, the amount of water is preferably
about 10 to 1000 times the mass of the bamboo fibers, more
preferably about 100 to 500 times, or still more preferably about
100 to 150 times. The mixing treatment is performed preferably at a
temperature of about 5 to 60.degree. C., or more preferably at a
temperature of about 5 to 40.degree. C. The operating conditions of
the mixer can be appropriately determined so as to achieve a
predetermined defibration effect.
[0076] The step (4) of removing hemicellulose from the defibrated
bamboo fibers can be performed by alkali treatment of the
defibrated bamboo fibers. The alkali treatment can be performed by
soaking the defibrated bamboo fibers in an aqueous alkaline
solution. The aqueous alkaline solution can be an aqueous potassium
hydroxide solution, or can also be an aqueous sodium hydroxide
solution or the like. When using an aqueous potassium hydroxide
solution, from the perspective of treatment efficiency, for every 5
g of fibers, about 50 to 500 ml or preferably about 200 to 500 ml
of a 0.5 to 5.0 M or preferably 1.0 to 2.0 M aqueous KOH solution
can be used. The soaking time is preferably 1 to 24 hours, more
preferably 1 to 12 hours, or still more preferably 1 to 8
hours.
[0077] The step (5) of removing metal components from the bamboo
fibers after hemicellulose removal can be performed by applying
acid treatment to bamboo fibers from which hemicellulose has been
removed. The acid treatment can be performed by contacting the
bamboo fibers with an acid solution and shaking for a predetermined
time. The acid solution can be an aqueous solution of hydrochloric
acid, perchloric acid, sulfuric acid, or nitric acid. For example,
when using a hydrochloric acid solution, the concentration of the
solution is preferably about 0.001 to 1.0 M, more preferably about
0.01 to 1.0 M, or still more preferably about 0.01 to 0.1 M. The
time of contacting is preferably 1 to 24 hours, more preferably 3
to 24 hours, or still more preferably 1 to 12 hours. The treatment
can be performed at room temperature (about 20 to 30.degree.
C.).
[0078] The amount of lignin in the bamboo fibers can be measured
according to, for example, a sulfuric acid method (The Japan Wood
Research Society, Wood Science Experiment Manual, pp. 96-97,
Bun-eido Publishing Co., Ltd. (2010)) (refer to the Examples
below).
[0079] The amount of hemicellulose in the bamboo fibers can be
measured based on the mass of bamboo fibers before and after
hemicellulose removal (refer to the Examples below).
[0080] The bamboo-derived cellulose nanofibers manufactured
according to the method of the present invention have the following
characteristics: a cellulose purity of not less than 90%, a fiber
diameter of 10 to 20 nm, and a crystallinity of not less than 70%.
The cellulose purity and the crystallinity of the cellulose
nanofibers of the present invention are significantly higher than
the cellulose purity (at most 87%) and the crystallinity (at most
66%) of conventional cellulose nanofibers.
[0081] By forming the bamboo-derived cellulose nanofibers according
to the present invention into a sheet, a sheet-like material
according to the present invention can be obtained. The sheet can
be formed by, for example, hot pressing or freeze-drying. Natural
drying can also be used.
[0082] Forming a sheet by hot pressing can be preferably performed
using a suspension obtained by stirring a to-be-treated solution
prepared by adding, to water, cellulose nanofibers after removal of
metal components. By removing the water as a dispersion medium from
the suspension, and, without drying, treating the residue collected
with by a hot press machine to form a sheet, a sheet-like material
comprising the cellulose nanofibers according to the present
invention can be obtained.
[0083] A suspension obtained by re-dispersing in an alcohol, such
as ethanol, as a dispersion medium, residue obtained by removing
water from a suspension can be used. When the dispersion medium is
water, aggregation of fibers is observed in the obtained sheet-like
material, and when the dispersion medium is alcohol, disaggregation
of fibers is observed due to solvation by the alcohol between
cellulose molecules.
[0084] Forming a sheet by freeze-drying can be performed preferably
using a suspension in which an organic solvent (an alcohol, for
example) is the dispersion medium. By spreading the suspension as a
film on a predetermined substrate, freezing it, and then applying
freeze-drying treatment, a sheet-like material comprising the
cellulose nanofiber according to the present invention can be
obtained. If the dispersion medium is an alcohol, the dispersion
medium can be ethanol. butanol or the like. If the dispersion
medium is an organic solvent other than an alcohol, the dispersion
medium can be a ketone (for example, acetone), an aromatic compound
(for example, toluene), a carboxylic acid (for example, acetic
acid), an amine (for example, N,N-dimethylformamide), or
acetonitrile. The dispersion medium (alcohol or the like) sublimes
due to the freeze-drying, whereby agglomeration of fibers can be
suppressed. The dispersion medium can be one type alone (for
example, ethanol), can be a mixture of multiple types, or can be
multiple types used sequentially (for example, after collecting
bamboo fibers from a suspension of ethanol, re-dispersing the
bamboo fibers in butanol to form a suspension and creating a
sheet-like material from the resultant suspension). The latter case
has the merit of suppressing aggregation of the cellulose
nanofibers.
[0085] Regardless of the method of forming a sheet, the stirring to
obtain the suspension can be performed, for example, using a
general mixer, or using ultrasonic waves. The amount of cellulose
nanofiber contained in the suspension is generally 0.1 to 10 wt %,
more preferably 0.1 to 2.0 wt %, or even more preferably 0.1 to 1.0
wt %. The stirring conditions are not particularly limited, as long
as a suspension in which cellulose nanofibers are sufficiently
dispersed is obtained. Removal of the water or alcohol of the
dispersion medium can be performed by any treatment such as
filtering.
[0086] Natural drying can be performed by dispersing the
bamboo-derived cellulose nanofiber, spreading the suspension into a
film over a substrate, and then letting the suspension sit such
that the dispersion medium is removed. The dispersion medium can be
water or an organic solvent such as an alcohol. In some cases, the
removal of dispersion medium can be accelerated by ventilation.
[0087] The sheet-like material created from the bamboo-derived
cellulose nanofiber of the present invention demonstrates improved
strength over a sheet-like material created with conventional
cellulose nanofiber, when measuring the strength under the same
conditions. For example, comparing tensile strengths relative to
200 g/m.sup.2 of basis weight (mass per m.sup.2 of sheet material),
the tensile strength of the sheet material according to the present
invention is about 200 N, whereas the tensile strengths of a sheet
material created from cellulose fibers FD100G obtained from Daicel
FineChem Ltd. and commercially available paper (ISO9707-certified
paper) obtained from Mondi Limited are about 100 N and 145 N,
respectively.
[0088] Thus, the sheet-like material comprising bamboo-derived
cellulose nanofibers of the present invention, which demonstrates
such a high tensile strength, is expected to be useful in fields
such as reinforcement, acoustics, medical treatment, food products,
packaging materials, transport and the like.
EXAMPLES
[0089] The present invention will now be further described by way
of Examples. Naturally, the present invention is not limited to the
following Examples.
[0090] 1. Preparation of Bamboo Fibers in Nanometer Size
[0091] The inner and outer skins were removed, and pressing
treatment was applied, to obtain chipped bamboo pieces with a
length of about 10 cm. 120 g of the bamboo pieces were placed in an
electric pressure cooker (Panasonic, SR-P37-N), soaked in 2 L of a
0.10 M aqueous sodium hydroxide solution, and treated for 3 hours
under conditions of 120.degree. C. at 200 kPa. After the treated
bamboo pieces were allowed to cool, the bamboo pieces were placed
in a metallic sieve and washed with ultrapure water until the
effluent water became neutral, whereby the bamboo fibers were
obtained. 60 g of the obtained bamboo fibers were placed in a mixer
(Vitamix.TM. ABS-BU), 1 L of ultrapure water was added, and the
mixture was stirred for 1 minute at 37,000 rpm. Thereafter, the
ultrapure water was removed, and the fibers were dried to obtain
bamboo fibers.
[0092] 2. Delignification Treatment
[0093] In a 300 ml glass Erlenmeyer flask, a 17.5 M acetic acid
solution was added, and a 11.6 M hydrogen peroxide solution was
slowly dripped therein using a separatory funnel to make 100 ml of
a peracetic acid solution. The volume ratio of acetic acid to
hydrogen peroxide was set to 1:1.
[0094] 10 g of the bamboo fibers obtained in above item 1 were
added, in a batch-wise manner, about 1 g at a time with mixing by a
glass rod into a container containing 100 ml of a peracetic acid
solution, and thereafter the temperature of a water bath (EYELA,
SB-350) was set to 80.degree. C., the cooling pipe which was
connected to the container was connected to a low temperature water
bath (EYELA, NCB-1200) and let sit for 1, 3, 6, or 8 hours while
under reflux. Thereafter, the container was allowed to cool, and
suction filtering was performed using plastic filters (ADVANTEC,
KP-47H and KP-47S). The residue was washed with ultrapure water
until the effluent water became neutral, and thereafter was dried
for 12 hours in a dryer kept at 60.degree. C., whereby delignified
bamboo fibers were obtained.
[0095] After 1 hour of treatment in the peracetic acid, the fiber
changed from a brown color to a yellow color, and after 3 hours of
treatment, the color was yellow-white. After 6 hours of further
treatment, white-colored bamboo fibers were obtained. This is
considered to correspond to the removal of the colored component
lignin as the peracetic treatment progresses. No changes were
observed after 6 or more hours of treatment.
[0096] 3. Defibration
[0097] Delignified bamboo fibers were added to ultrapure water to
make its concentration of 0.7 wt %, and the mixture was stirred for
5 minutes at 37,000 rpm using a mixer (Vitamix.TM. ABS-BU).
Thereafter, the mixture was allowed to cool, and then mixed
intermittently for a total of 60 minutes, whereby a suspension of
defibrated bamboo fibers was obtained.
[0098] 4. Quantitative Determination of Lignin
[0099] 15 ml of 13.4 M sulfuric acid and 1 g of the product
(delignified bamboo fibers) obtained from above item 2 were added
into a 100 ml glass beaker, and the mixture was stirred with a
glass rod until the sulfuric acid evenly infiltrated the fibers.
After letting the mixture sit for 4 hours, it was boiled for 4
hours under reflux and then allowed to cool. Thereafter, the
residue was collected after suction filtering using a glass filter
(Sibata Scientific Technology, 1GP16), and was washed with 500 ml
of hot water and then dried for 12 hours in a dryer kept at
105.degree. C. After drying, the yield was quantitatively measured
to 4 decimal places, and the content of lignin was found using the
following equation (1) (see The Japan Wood Research Society, Wood
Science Experiment Manual, p. 97, Bun-eido Publishing Co., Ltd.
(2010)).
Lignin content (wt %)=(mass after experiment/mass before
experiment).times.100 (1)
[0100] The content and extraction rate of lignin relative to
treatment time in peracetic acid are shown in Table 1, and the
graph thereof is shown in FIG. 1. The longer the time of peracetic
acid treatment, the larger the extraction rate, such that at 6
hours, the entire amount of lignin was extracted.
TABLE-US-00001 TABLE 1 Extrac- Peracetic Lignin content (wt %) tion
acid Sample Sample Sample Aver- Standard rate treatment 1 2 3 age
deviation (%) Before 30.7 30.1 29.6 30.1 0.45 -- treatment 1 hour
0.94 1.17 1.21 1.11 0.12 96.3 3 hours 0 0 0 0 0 100 6 hours 0 0 0 0
0 100 8 hours 0.23 0 0 0.08 0.11 99.7
[0101] In the case of obtaining nanofibers including a small amount
of residual lignin, which is known as so-called "lignocellulose
nanofibers", bamboo fibers (including about 1 wt % of residual
lignin) obtained after about 1 hour of treatment can be further
treated in accordance with the procedures explained below.
[0102] 5. Quantitative Determination of Hemicellulose
[0103] According to a reference literature, .beta.-cellulose,
.gamma.-cellulose, and hemicellulose are categorized as
hemicellulose, and all others as .alpha.-cellulose (see The Japan
Wood Research Society, Wood Science Experiment Manual, p. 95,
Bun-eido Publishing (2010)). In the present invention, accordingly,
the quantification method for .alpha.-cellulose was used to measure
hemicellulose (see The Japan Wood Research Society, Wood Science
Experiment Manual, pp. 96-97, Bun-eido Publishing Co., Ltd.
(2010)). Therefore, the hemicellulose here includes .beta.- and
.gamma.-celluloses.
[0104] 25 ml of a 5.80 M aqueous sodium hydroxide solution and 1 g
of the product (delignified bamboo fibers) obtained from above item
2 were added into a 200 ml plastic beaker. Once the liquid evenly
infiltrated the fibers, the mixture was let sit for 4 minutes, then
stirred using a plastic stirring rod for 5 minutes, and let sit for
30 minutes. Ultrapure water was added to the beaker and stirred for
1 minute, and then let sit for 5 minutes. Thereafter, suction
filtering was performed using a glass filter (Sibata Scientific
Technology, 1GP250), the filtrate was collected and re-filtered,
and then the residue was washed with ultrapure water until the
filtrate was neutral. The residue and 40 ml of 1.75 M aqueous
acetic acid solution were added to a 100 ml glass beaker, and then
let sit for 5 minutes. The residue was then collected by suction
filtering, and washed with 1 L of ultrapure water. Thereafter, the
residue was dried for 12 hours in a dryer kept at 105.degree. C.,
and quantitatively measured to 4 decimal places. The content of
hemicellulose was found using the following equation (2) (see The
Japan Wood Research Society, Wood Science Experiment Manual, p. 96,
Bun-eido Publishing Co., Ltd. (2010)).
Hemicellulose content (wt %)=((mass before experiment-(mass of
.alpha.-cellulose))/mass before experiment).times.100 (2)
[0105] The content and extraction rate of the hemicellulose of the
bamboo fibers after peracetic acid treatment are shown in Table 2,
and the graph thereof is shown in FIG. 2. The hemicellulose content
did not greatly decrease in the peracetic acid treatment.
Additionally, no great changes due to treatment time were
observed.
TABLE-US-00002 TABLE 2 Extrac- Peracetic Hemicellulose content (wt
%) tion acid Sample Sample Sample Aver- Standard rate treatment 1 2
3 age deviation (%) Before 20.8 21.8 21.3 21.3 0.41 -- treatment 1
hour 19.2 17.3 18.7 18.4 0.80 13.6 3 hours 13.6 13.2 17.6 14.8 1.99
30.5 6 hours 15.6 15.8 16.8 16.1 0.52 24.4 8 hours 17.8 16.0 17.3
17.1 0.76 19.7
[0106] 6. Removal and Quantitative Determination of
Hemicellulose
[0107] 5 g of bamboo fibers which had undergone delignification
treatment for 6 hours using peracetic acid and defibration
treatment were placed in a 200 ml glass Erlenmeyer flask, to which
200 ml of a 0.71 M or 1.18 M aqueous potassium hydroxide solution
was added. The solution was allowed to evenly infiltrate the
fibers. The flask was sealed with a stopper, and let sit at room
temperature for 12 hours. Then the solution underwent suction
filtering using a plastic filter (ADVANTEC, KP-47H and KP-47S), and
the residue was washed using ultrapure water until the effluent
water became neutral. Thereafter, the residue was dried for 12
hours in a dryer kept at 60.degree. C., and the content of
hemicellulose was measured using the same procedure and formula as
in above item 5. The results are shown in Table 3 and the
relationships between treatment solution concentration and
hemicellulose content and extraction rate are shown in FIG. 3. With
a 1.18 M solution, about 7% hemicellulose was included. The content
of .alpha.-cellulose was about 93%.
TABLE-US-00003 TABLE 3 Extrac- Hemicellulose content (wt %) tion
KOH Sample Sample Sample Aver- Standard rate treatment 1 2 3 age
deviation (%) Before 15.6 15.8 16.8 16.1 0.52 -- treatment 0.71 M
17.9 18.1 17.0 17.7 0.48 16.9 1.18 M 7.37 6.67 7.24 7.09 0.30
66.7
[0108] In order to clarify the relationship between the volume at
the time of treatment with potassium hydroxide and extraction rate,
while the concentration in the aqueous KOH solution was kept at
1.18 M, the volume of the solution was changed. The quantitative
results for the cases of using 100 ml and 200 ml of solution are
shown in Table 4. Additionally, the relationship between the
solution volume and the content and extraction rate of
hemicellulose is shown in FIG. 4. From the results, it is clear
that the most hemicellulose was extracted when 200 ml of a 1.18 M
aqueous KOH solution was used for 5 g of bamboo fibers.
TABLE-US-00004 TABLE 4 Extrac- Content (wt %) tion KOH Sample
Sample Sample Aver- Standard rate treatment 1 2 3 age deviation (%)
Before 15.6 15.8 16.8 16.1 0.52 -- treatment 100 ml 11.3 12.3 11.6
11.7 0.42 44.1 200 ml 7.37 6.67 7.24 7.09 0.30 66.7
[0109] Next, the relationship between treatment temperature and the
content and extraction rate of hemicellulose was investigated. The
measured results are shown in Table 5 and FIG. 5. When the aqueous
KOH solution concentration was 1.18 M and 200 ml of a solution was
used for 5 g of bamboo fibers, more hemicellulose was extracted
when treated at temperatures higher than 100.degree. C. When the
treatment temperature was 100.degree. C., the bamboo fibers and an
aqueous KOH solution were added into a Teflon.TM. container. The
container was placed in a heat-resistant stainless steel container,
which was then sealed. The dryer temperature was set to 100.degree.
C., and the contents were left to sit for 12 hours. After allowing
the containers to be cooled, the contents were suction filtered,
washed and dried as described above.
TABLE-US-00005 TABLE 5 Extrac- Content (wt %) tion Treatment Sample
Sample Sample Aver- Standard rate temperature 1 2 3 age deviation
(%) Before 15.6 15.8 16.8 16.1 0.52 -- treatment Room 7.37 6.67
7.24 7.09 0.30 66.7 temperature 100.degree. C. 11.3 11.2 11.3 11.3
0.05 46.9
[0110] 7. Quantitative Determination of .beta.-Cellulose
[0111] The content of .alpha.-cellulose in the product treated in
the optimal conditions is about 93%.
[0112] In order to accurately find the amount of .beta.-cellulose
in the product, the product was added to 10 ml of a 30% acetic acid
solution, to which 200 ml of the washing liquid obtained in above
item 5 was added. Then the solution was heated to 80.degree. C.,
kept at that temperature, and left to sit for 9 hours. The obtained
precipitate was collected in pre-weighed filter paper. The increase
in mass after drying was considered to be the content of
.beta.-cellulose (see The Japan Wood Research Society, Wood Science
Experiment Manual, p. 96, Bun-eido Publishing Co., Ltd.
(2010)).
TABLE-US-00006 TABLE 6 Content (wt %) Sample 1 Sample 2 Sample 3
Average Hemicellulose 7.37 6.67 7.24 7.09 .beta.-cellulose 7.3 6.4
7.0 6.9 .alpha.-cellulose 92.2 93.6 92.8 92.9 Total of .beta.- and
99.5 100 99.8 99.8 .alpha.- celluloses
[0113] From the results in the third and fourth rows from the top
of Table 6, it was found that 97% of the hemicellulose measured by
the method described in above item 5 is .beta.-cellulose. Combined
with the total for .alpha.-cellulose content, the cellulose content
was confirmed to be 99.8%.
[0114] 8. Morphological Observation Via Field Emission Scanning
Electron Microscope
[0115] 1 mL of ethanol was added to one drop of suspension of the
bamboo fibers (before delignification) obtained in above item 1,
and then the fibers were ultrasonically dispersed. 10 .mu.L of the
suspension after ultrasonic dispersion was dripped onto glassy
carbon, and dried in a dryer kept at 60.degree. C. Thereafter,
platinum was vapor deposited onto the dried bamboo fibers using a
vapor deposition apparatus (JEOL Ltd., JFC-1600), and then the
morphology of the bamboo fibers was observed via a field emission
scanning electron microscope (FE-SEM (JEOL Ltd., JSM-6701F)). The
vapor deposition conditions are shown in Table 7 and the
measurement conditions are shown in Table 8. Additionally, the
bamboo fibers which underwent the delignification treatment
described in above item 2 for 1, 3, 6, and 8 hours were likewise
observed. The respective observation results are shown in FIGS. 6
(a) to 10 (a) (FE-SEM images) and FIGS. 6 (b) to 10 (b) (fiber
distribution).
TABLE-US-00007 TABLE 7 Sputtering current 20 mA Vapor deposition
time 15 s Number of vapor deposition events 2
TABLE-US-00008 TABLE 8 Acceleration voltage 3 kV Working distance 3
mm Irradiation current 7 .mu.A Emission current 10 .mu.A
[0116] When observing at high magnification using an FE-SEM, it was
found that the short fiber (not shown) having a diameter of about
16 .mu.m as confirmed by optical microscope observation of bamboo
fibers before delignification treatment was a bundle of tangled
fibers having diameters within a wide range (FIGS. 6 (a) and (b)).
The longer the delignification treatment time, the smaller the
diameter of fibers. The particulate substance seen in the FE-SEM
image is vapor deposited platinum.
[0117] After 8 hours, the fiber diameters were 15.9 nm on average.
In general, the diameter of the cellulose nanofibers contained in
wood material or the like is several nanometers, and therefore the
above fiber diameters were somewhat thicker than that. This is
because samples for FE-SEM observation must be completely dry, and
there was a possibility that during the drying, hydrogen bonding
between the cellulose molecules led to association of the
molecules.
[0118] Next, similar observation via FE-SEM was performed for
bamboo fibers (bamboo fibers treated using 1.18 M aqueous potassium
hydroxide solution indicated in above item 6) from which
hemicellulose as well as lignin was removed. The results are shown
in FIG. 11 (a) (FE-SEM image) and FIG. 11 (b) (fiber distribution).
Even for fibers from which hemicellulose was removed, a portion of
bundles remained, but the average fiber diameter was small, and it
is likely that hemicellulose extraction is related to the diameter
of cellulose nonafibers produced.
[0119] 9. Qualitative Analysis Via Fourier Transform Infrared
Spectroscopy
[0120] Respective bamboo fibers after peracetic acid treatment
(delignification treatment) for 1, 3, 6, and 8 hours and bamboo
fibers for which hemicellulose removal was performed after 8 hours
of peracetic acid treatment were placed in a mixer (Vitamix.TM.
ABS-BU), ultrapure water was added, and the mixture was stirred for
60 minutes. Thereafter, the mixture was suction filtered using a
plastic filter (ADVANTEC, KP-47H and KP-47S), and dried in a dryer
kept at 60.degree. C. An FT-IR spectra in the range of 4000 to 550
cm.sup.-1 was measured for the obtained bamboo fiber sheet using a
Fourier transform infrared spectrometer (FT-IR (Thermo Fisher
SCIENTIFIC, ART iD5)) equipped with a diffuse reflection unit. The
results are shown in FIG. 12.
[0121] The peaks of the CO stretch at 1760 cm.sup.-1, aromatic
C.dbd.C stretch at 1500 cm.sup.-1, CO antisymmetric stretch of a
methoxy group at 1250 cm.sup.-1, and aromatic CH stretch at 840
cm.sup.-1, which are attributable to lignin confirmed in the FT-IR
spectra (not shown) of the raw material bamboo fibers before
peracetic acid treatment (delignification treatment), were not
observed, but the peaks of the OH stretch at 3600 to 3000 cm.sup.-1
and the CH stretch at 2920 cm.sup.-1 which are attributable to
cellulose and hemicellulose were confirmed.
[0122] 10. Removal of Metal Components and Qualitative and
Quantitative Analysis of Metals Via ICP Emission Spectroscopy
[0123] The bamboo fibers obtained in above item 2 (bamboo fibers
with the lignin removed) were contacted with an aqueous
hydrochloric acid solution, to thereby remove metal components. 1 g
of bamboo fibers and 50 ml of a 0.01 M aqueous hydrochloric acid
solution were added into a plastic sample tube. While leaving the
bamboo fibers behind, the solution was quickly extracted from the
sample tube to obtain solution "before treatment". 50 ml of an
aqueous hydrochloric acid solution was newly added to the sample
tube, and after 24 hours of shaking the tube, the solution was
quickly extracted leaving the bamboo fibers behind to obtain
solution "after treatment". Each of the solutions was added into a
10 ml glass screw top test tube, treated with a centrifuge (AS ONE
Corporation, C-12B) such that the solid fraction precipitated, and
then the supernatant liquid was removed. The supernatant liquid was
analyzed using an inductively coupled plasma optical emission
spectrometer (ICP-OES (Agilent Technologies, 710 ICP-OES)), and
qualitative and quantitative analysis was performed on the metals
contained in the bamboo fiber. The results are shown in Table
9.
TABLE-US-00009 TABLE 9 Content (%) Element Al Ba K Mg Mn Zn Before
0 0 0.356 0 0 0.014 treatment After 0 0 0.005 0 0 0.054
treatment
[0124] Bamboo is known to contain mainly silica (silicon oxide),
calcium, potassium, magnesium, and sodium in large amounts. In the
bamboo fiber of the present invention analyzed here, potassium and
zinc were confirmed to be present, but other metals were present in
extremely minor amounts at nearly 0%. It was found that according
to the present invention, metals can be removed by soaking the
fibers in hydrochloric acid for 24 hours, and the content of metals
can be reduced to about 0.06% relative to the mass of obtained
cellulose nanofiber.
[0125] 11. Creation of Cellulose Nanofiber Sheet by Hot
Pressing
[0126] 3.5 g of bamboo fibers reacted with peracetic acid solution
for 6 hours in the procedure explained above in above item 2 were
added to 500 ml of ultrapure water to make a fiber content of 0.7
wt %, the solution was stirred for 5 minutes at 37,000 rpm using a
mixer (Vitamix.TM. ABS-BU), and after allowing it to be cooled, the
solution was stirred intermittently for a total of 60 minutes to
obtain a suspension. The obtained suspension was suction filtered
using a glass filter (ADVANTEC, KG-47), and thereafter, without
drying, was pressed at 120.degree. C. using a small heat press
machine (AS ONE Corporation, AH-2003) to create a sheet.
Additionally, after filtering the suspension obtained above, the
residue was added to 100 ml of ethanol, ultrasonically dispersed
and then suction filtered. After this was repeated two times, a
sheet was created using the small heat press machine in a similar
method.
[0127] 12. Creation of Cellulose Nanofiber Sheet by
Freeze-Drying
[0128] After filtering the suspension obtained in above item 11,
the suspension was added to 50 ml of ethanol, ultrasonically
dispersed, and suction filtered. After this was repeated 2 times,
the residue was added to 50 ml of t-butyl alcohol and
ultrasonically dispersed. This process was also repeated 2 times,
and after filtering, the residue was transferred to a petri dish,
frozen in a freezer (Panasonic, NR-B175W), and dried using vacuum
freeze-drying equipment (Hitachi Corp., ES-2030) to create a sheet.
The drying conditions are shown in Table 10. For the sake of
comparing morphology, a water suspension was also frozen directly
in the freezer and freeze-dried to create a sheet.
TABLE-US-00010 TABLE 10 Setting temperature -10.degree. C. Degree
of vacuum at completion of drying Less than 0.1 Torr (13.3 Pa)
[0129] 13. Observation of Morphology of Sheets Via Field Emission
Scanning Electron Microscope and Transmission Electron
Microscope
[0130] The morphology of the obtained cellulose nanofiber sheet was
observed via a field emission scanning electron microscope
(FE-SEM). The measurement conditions are shown in Table 11.
Platinum was vapor deposited on the sheet using a vapor deposition
apparatus (JEOL Ltd., JFC-1600) before observation. The vapor
deposition conditions are shown in Table 12.
TABLE-US-00011 TABLE 11 Acceleration voltage 3 kV Working distance
3 mm Irradiation current 7 Emission current 10 .mu.A
TABLE-US-00012 TABLE 12 Sputtering current 20 mA Vapor deposition
time 15 s Number of vapor deposition events 2
[0131] Additionally, the created sheet was ultrasonically dispersed
in 1-butanol. The dispersion was dripped onto a TEM grid (Okenshoji
Co., Ltd., STEM 150 Cu grid), dried by a dryer kept at 100.degree.
C., and then the sheet morphology was observed via a transmission
electron microscope (TEM) (JEOL Ltd., JEM-2100).
[0132] FIG. 13 (a) shows a TEM image of a cellulose nanofiber sheet
created using a hot press with water as a dispersion medium, FIG.
13 (b) shows a picture of the external appearance and an FE-SEM
image thereof, and FIGS. 14 (a) and (b) show the respective images
for a sheet created using a hot press with ethanol as a dispersion
medium.
[0133] In the case of water as the dispersion medium, it is
confirmed that the fibers aggregate, which is considered to be due
to the strong hydrogen bonds between cellulose molecules. When the
dispersion medium was changed to ethanol, slight dispersion of the
fibers was confirmed, which is considered to be due to ethanol
entering the gap between cellulose molecules and solvating them
such that the cellulose molecules disaggregate.
[0134] FIG. 15 (a) shows a TEM image of the cellulose nanofiber
sheet created by freeze-drying, and FIG. 15 (b) shows a picture of
the external appearance and an FE-SEM image thereof. It was
confirmed that by freeze-drying, the fibers do not aggregate and
are disaggregated compared to sheets created by hot press (FIG. 13
(a), (b) and FIG. 14 (a), (b)). This is considered to be due the
fact that fiber aggregation is suppressed because the dispersion
medium t-butyl alcohol directly sublimates, without passing through
a liquid state, from a solid state to a gaseous state due to
freeze-drying.
[0135] 14. Evaluation of Crystallinity of Sheets Via XRD
[0136] The crystallinities of obtained cellulose nanofiber sheets
were evaluated using an X-ray diffractometer (XRD (Rigaku
Corporation, RINT-Ultima III)). The measurement conditions are
shown in Table 13.
TABLE-US-00013 TABLE 13 X-ray tube Cu Acceleration voltage 30 kV
Tube current 16 mA Divergence slit 0.5 degrees Scattering slit 0.5
degrees Receiving slit 0.15 mm Sampling range 0.05 degrees Scanning
speed 5 degrees/min Measuring range 2.theta. = 10 to 80 degrees
[0137] Additionally, the crystallinity of cellulose was calculated
using a strength (I.sub.A) of the 10-1 diffraction peak of
cellulose at 2.theta.=15.degree. from the baseline drawn from
2.theta.=10.degree. to 80.degree. and a strength (I.sub.B) from the
baseline drawn from 2.theta.=10.degree. to 20.degree., and using
the following equation (3).
Crystallinity=(I.sub.A/I.sub.B).times.100 (3)
[0138] The XRD patterns for 3 samples of cellulose nanofiber sheets
obtained from 3 samples are shown in FIGS. 16 to 18. In each of the
samples, the 10-1 and 002 diffraction peaks of cellulose were
confirmed at 2.theta.=15.degree. and 22.5.degree.. Additionally,
the crystallinities of the celluloses calculated based on these
peak strengths are shown in Table 14. The crystallinities were 71
to 77% regardless of dispersion medium.
TABLE-US-00014 TABLE 14 Method for manufacturing sheet
Crystallinity (%) Hot press, water dispersion medium 75.0 Hot
press, ethanol dispersion medium 71.4 Freeze-drying, butanol
dispersion medium 77.5
[0139] 15. Measurement of Surface Area According to Gas
Adsorption-Desorption Measurement
[0140] The BET surface area of the obtained cellulose nanofiber
sheet was measured using a nitrogen gas adsorption/desorption
device (Yuasa Ionics, AUTOSORB-3). Nitrogen (99.9% purity) was
adsorbed on the sheet in a cell at 77 K, and the adsorption amount
and pressure inside the cell were measured to obtain an adsorption
isotherm. The obtained adsorption isotherm was analyzed using a BET
method to calculate the BET surface area. The sample was deaerated
by evacuation at 200.degree. C. for 24 hours before the
measurement. The measured gas adsorption/desorption isotherm is
shown in FIG. 19, and the BET surface area is shown in Table
15.
TABLE-US-00015 TABLE 15 Method for manufacturing sheet BET surface
area (m.sup.2/g) Hot press (HP), water dispersion medium 3.18 Hot
press (HP), ethanol dispersion medium 22.6 Freeze-drying, t-butanol
dispersion medium 36.0
[0141] In the case of a sheet created by hot pressing using ethanol
as a dispersion medium, the fibers experienced disaggregation such
that the surface area was larger than the case with water as the
dispersion medium. In the sheet created by freeze-drying,
aggregation of fibers was prevented, and thus, an even larger
surface area was obtained relative to the sheets created by hot
pressing.
[0142] Additionally, FIG. 20 (sheet created by hot pressing using
water as a dispersion medium), FIG. 21 (sheet created by hot
pressing using ethanol as a dispersion medium), and FIG. 21 (sheet
created by freeze-drying) show distributions of micropore sizes in
the sheets.
[0143] 16. Measurement of Strength by Tensile Test
[0144] The tensile strength of a cellulose nanofiber sheet created
by hot pressing using water as a dispersion medium was measured.
The created sheet was cut into rectangular strips 1.5 cm wide by
2.5 cm long. The top and bottom of the strip were clipped to a
length of about 5 mm, and a tensile strength was measured using a
desktop precision universal tester (SHIMADZU, AGS-J) at a tensile
speed of 1 mm/min. For comparison, tensile strengths of a sheet
formed by hot pressing food product-use cellulose nanofibers
(Celish.TM.) produced by Daicel FineChem Ltd. and
commercially-available paper (ISO9707-certified paper, low residual
lignin) from Mondi Limited were measured in the same way.
[0145] FIG. 23 shows the maximum strength relative to the mass of
each sample, FIG. 24 shows the maximum strength relative to the
thickness of each sample, FIG. 25 shows the maximum strength
relative to the density of each sample, and FIG. 26 shows the
maximum strength relative to the basis weight (mass per m.sup.2) of
each sample. For each sample, it was confirmed that as mass,
thickness, density or basis weight increased, the strength also
increased.
[0146] Additionally, of the three types of sheets tested, the
bamboo-derived cellulose nanofiber sheet of the present invention
demonstrated the highest strength. This is considered to be because
the bamboo-derived cellulose nanofibers according to the present
invention are finer than other fibers, such that the number of
fibers per amount of mass is higher, and the sites for hydrogen
bonding between fibers increases, thereby increasing the strength.
For example, when comparing the FE-SEM images and fiber diameter
distributions obtained from the sheet of bamboo-derived cellulose
nanofibers according to the present invention and the sheet created
in the same manner using cellulose nanofiber Celish.TM.
(respectively shown in FIGS. 27 (a) and (b) and FIGS. 28 (a) and
(b)), it can be confirmed that the fibers of the former are
thinner.
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